I started an installation of the Xcode 4 developer's preview last night before going to bed. When I woke up, the installation had popped up a message saying to close Xcode first (doh!). I did so, but whatever script had displayed the message seemed to have hung because the (uncancellable) message wouldn't disappear, and the installer wouldn't progress.
After force quitting the Installer app, the message above was still showing, so I restarted my MacBook Pro and then tried the installation again. This time it wouldn't even start, instead showing one of those candy-stripe activity bars and the message:
Waiting for other installations to finish
At this point, I'm stumped. There are no other installations in progress, so there must be some artifact left over from the interrupted install that's gumming up the works. But what?